UV-hardenable and thermally hardenable epoxy resins for underfilling electrical and electronic components

ABSTRACT

UV-hardenable and thermally hardenable epoxy resins for the underfilling process in electrical and electronic components are provided. A new casting resin component has an increased storage stability at room temperature of at least 6 months and improved characteristics for the underfilling process including faster hardening and lower viscosity. Filler material with a maximum grain size of 20 μm and a siloxane component are provided in addition to the casting resin formulation.

BACKGROUND OF THE INVENTION

The invention relates to a sealing material for the underfilling ofelectronic and electrical components, which are sealed in anunderfilling process for protection from environmental influences and toimprove the resistance to fatigue of the solder connections of thecomponents.

Sealing materials are known that consist of two-component epoxy resins,and that either must be mixed before processing or must be stored as apre-mixture at -40° C. At room temperature, they are available only fora few hours before they harden and become unusable. Disadvantages ofthis prior art include, on the one hand, the additional procedural stepof the mixing of the two components before processing, and on the otherhand the storage at low temperatures such as -40° C. (see "Encapsulantsused in Flip-Chip Packages," D. Suryanarayana, T. Y. Wu, J. A. Varcoe,IBM Microelectronics, IEEE Transact., Hybr., Manuf. Techn., Vol. 16, No.8.858; 1993, and "Underfilling for Flip-Chip Mounting," M. Norris,Camelot Systems S. A. Conference proceedings "Adhesives in Electronics,"Berlin 1994).

There is thus a need for sealing materials of this type that do not haveto be mixed before processing and do not require storage at lowtemperatures.

SUMMARY OF THE INVENTION

It is an advantage of the present invention to provide a sealingmaterial that can be stored at room temperature for a longer period oftime and that is suitable for use as a sealing material for theunderfilling technique for the protection of components fromenvironmental influences and for the improvement of the resistance tofatigue of the contained soldered connections.

The present invention provides a mixture of an epoxy resin, a siloxane,and a filler material (whereby the filler material may not exceed acertain grain size) forms a three-phase system that is stable in theliquid state, in which the filler material is homogeneously distributedand epoxy/siloxane form a stable emulsion.

In an embodiment, the present invention is a casting resin formulationcomprising the following components: an epoxy resin; a siloxane; afiller material with a maximum grain size less than 20 μm; a photoinitiator; and a thermal initiator.

In addition, the present invention is directed towards the use of such acasting resin formulation in the underfilling of electronic components.

Other objects and advantages of the present invention will becomeapparent upon reading the following detailed description and appendedclaims, and upon reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a sectional view of a substrate and chip with a gap disposedtherebetween in which the sealing compound of the present invention isinserted under capillary action.

It should be understood that the drawing is not necessarily to scale andthat the embodiment is illustrated diagrammatically and in a fragmentaryview. Details which are not necessary for an understanding of theinvention or which render other details difficult to perceive may havebeen omitted. It should be understood, of course, that the invention isnot necessarily limited to the particular embodiment illustrated herein.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

In the inventive epoxy resin formulation, a composition of thecomponents in the following ranges (indicated in weight percent) hasproven advantageous, whereby the narrower ranges are always ranges thatare preferred in relation to the broader ones:

epoxy component: between 15 and 75 percent or preferably between 18 and65 percent or more preferably between 30 and 50 percent;

filler material: between 25 and 85 percent or preferably between 30 and80 percent or more preferably between 50 and 70 percent;

siloxane component: between 0.2 and 15 percent or preferably between 1and 10 percent or more preferably between 2 and 8 percent;

photoinitiator: between 0.01 and 5 percent or preferably between 0.05and 2 percent or more preferably between 0.1 and 1 percent; and

thermal initiator: between 0.01 and 5 percent or preferably between 0.05and 2 percent or more preferably between 0.1 and 1 percent.

In addition, it has proven advantageous if, as the epoxy resincomponent, cycloaliphatic epoxy resin is used, preferably particularlypure, i.e., with a low ion content.

In addition, it has proven advantageous to use quartz material as fillermaterial; quartz powder and/or Al₂ O₃ is thereby particularlyadvantageous.

In addition, it is advantageous to use as the siloxane component anIsocyanuratesiloxane component, as specified in EP-PS 0 399 199 (whosecontent is hereby incorporated by reference).

In particular, this concerns the lsocyanuratesiloxane-containingorganosilic compounds of the general formula ##STR1## wherein Q=--(CH₂)₃SiR₂ O(SiR₂ O)_(n) SiR₂ R', n is a whole number from 0 to 25 and x is awhole number from 0 to 10, and for the radicals R and R', which mayrespectively be equal or different:

R is selected from the group consisting of alkyl, cycloalkyl, aryl,aralkyl or alkaryl, and R' is selected from the group consisting of anepoxy-functional radical with 4 to 10 C-atoms or a (meth)acrylate-functional radical with at least 6 C-atoms.

The alkyl, cycloalkyl, aryl, aralkyl or alkaryl groups of R and R¹ maybe unsubstituted or substituted. The following groups are cited asexamples for the radical R.

alkyls with 1 to 4 C atoms, such as methyl, ethyl, propyl, isopropyl,butyl and isobutyl, whereby the methyl radical is preferred;

cycloalkyls with 5 to 8 C atoms, such as cyclohexyl , methylcyclohexyland cycloheptyl;

aryls with 6 to 10 C atoms, such as phenyl and napthtyl;

aralkyls, such as β-phenylethyl, β-phenylpropyl, o-methylphenylethyl,3,5-deibromophenylethyl, p-nonylphenylethyl, o-bromophenylethyl, 3,5,deibromophenylethyl, p-chlorphenylethyl and 3,5-deichlorphenylethyl;alkaryls, such as tolyl.

The epoxy-functional radicals R' comprise 4 to 10 C atoms whereby theepoxy group is connected to the siloxane chain via a carbon bridge,which can also contain hetero-atoms. These radicals R' can be derivedfrom vinyl-functional epoxies or allyl-functional epoxies in such a waythat the vinyl function or, respectively, allyl-function is added to anSi-H-function. As examples, the following compounds are named asvinyl-functional or, respectively, allyl-functional epoxies:allylglycidylether, 4-vinylcyclohexenoxide, 4-vinylnorbomenoxide and1,2-epoxy-3-butene.

The (Meth)-acrylate-functional radicals R', in which the (Meth) acrylategroup is likewise bound to the siloxane chain via a carbon bridge, canbe derived from the epoxy-functional radicals, in such a way that theepoxy ring is opened by means of (meth) acrylate acid. In this case, the(Meth) acrylate-functional radicals comprise 7 to 14 C atoms. However,the (Meth) acrylate groups of the (meth) acrylate-functional radicals R'can also be bound to a carbon bridge to the siloxane chain in the mannerof an ester or via a carbonate grouping or urethane grouping.

Organosilicon compounds of the following type are thereby preferred:

compounds in which the radicals R' are epoxy-functional radicals thatcan be derived from the following unsaturated epoxies:1,2-epoxy-3-butene, 1,2-epoxy-5-hexene, 1,2-epoxy-7-octene,allylglycidylether, r-vinylcyclohexenoxide, 4-vinylnorornenoxide,norbornadienoxide, limonene oxide and dicyclopentadieneoxide;

compounds in which the radicals R' are (meth) acrylate-functionalradicals, produced by ring opening of the epoxy radical of compounds ofthe type named above with (meth) acrylate acid;

compounds, in which the radicals R' are (meth) acrylate-functionalradicals, which are bound to a carbon bridge to the siloxane chain inthe manner of an ester or via

a carbonate grouping or urethane grouping.

The compounds are polymerizeable organosilicon compounds containingisocyanurate, and comprising epoxy functions and/or (meth) acrylatefunctions. By means of the invention, liquid pre-polymers are thuscreated, in whose chemical construction rigid structure elements andflexible structure elements are combined, and which comprise reactivegroups, via which a polymerization (of the pre-polymers) can result.

A further development of the invention is that a filler material with amaximum grain size smaller than 15 μm or preferably smaller than 10 μmis used as the filler component.

The term underfilling process refers to a method for sealing a gapbetween chip and substrate in which the sealing material is dosed alongthe edge of the chip in front of the gap and flows under this gap bymeans of capillary forces. In general, the chips have an edge length of5 to 20 mm, and the gap has a height of 10 to 80 μm. It is therebynecessary that the gap between the substrate and the chip be filled withthe resin in a suitable time. In a short time, the resin should hardensufficiently that a problem-free handling of the component is ensured.The hardened resin should be provided with suitable physical properties,such as low stress and low expansion coefficients and high chemicalpurity and good resistance to chemicals.

The inventive reaction epoxy mixtures are one-component systems that canbe stored for several month at room temperature. During the storagetime, use characteristics such as viscosity, flow behavior or hardeningprofile do not change, nor do the final properties of the hardened resinsuch as for example bonding and thermal mechanical properties.

The inventive casting resin formulation can be hardened either by meansof UV radiation or thermally. Thus, for example an underfilled chip canbe hardened at the edges, where the reaction resin can be reached by theUV radiation, by means of a short UV flash to such an extent that thecomponent can be handled without difficulty. The shadowed region underthe chip can be hardened by a thermal process that can ensue at a latertime.

For the epoxy resin component of the casting resin formulation, epoxyresins and arbitrary mixtures of different epoxy resins are used. Here,the term "epoxy resin" correspondingly includes both homogenous epoxyresins and also mixtures of different epoxy resin. Epoxy resins that arecationically hardenable and that are free of solvent are therebypreferably used. The use of cycloaliphatic epoxies is therebyadvantageous. These are available with a high degree of purity, i. e.,with a low ion content, and thus with a low content of materials thatpromote corrosion. Commercially available cycloaliphatic epoxies alsoinclude mixtures of several cycloaliphatic epoxies. Suitable compoundsare given for example by the following structural formulas. ##STR2##However, it is also possible to use mixtures of different epoxy resins,or, respectively, components containing epoxy groups. Aside from thealready-named epoxy resins, glycidylethers, commonly used in the art,are suitable for use as epoxy resins in the present invention.

The term "siloxane" refers according to the invention to pure siloxanesand/or also to mixtures of different siloxanes. As the siloxanecomponent, polydimethylsiloxanes can be used, which during hardeningtend to phase separation from the epoxy matrix, thereby formingmicro-domains at whose boundary surfaces tensions can be reduced.According to the present invention, isocyanuratesiloxane, as specifiedfor example as the siloxane component in EP-PS-0 399 199, is therebypreferably used. Mixtures of different siloxanes can also be used.

As the filler component, all filler materials standard in theformulation of epoxy resins with bonding agents, coated or uncoated, canbe used, provided that they have the maximum grain size of less than orequal to 20 μm required for the invention. According to the invention,filler materials with maximum grain sizes of less than or equal to 15 μmare preferably used, advantageously less than or equal to 10 μm, and,particularly preferably, those whose maximum grain size does not exceed7 μm. The type and external shape of the contained quartz materialfiller particles are arbitrary, as long as the maximal size of thegrains lies within the indicated boundaries. Spherical particles are tobe preferred. Reaction resins filled in this way yield the inventivegood flow properties in the underflowing of the chip by means ofcapillary action.

In order to induce cationic hardening, a cationic photo initiator or acationic photo initiator system is incorporated into the composition ofthe present invention.

Upon UV radiation, these photo initiators release reactive cations,e.g., protons, which initiate the cationic hardening process of theepoxy resin. The photo initiators are thereby largely derived fromorganic onium salts, in particular those with nitrogen, phosphorous,oxygen, sulfur, selenium, or iodine as the central atom of the cation.Aromatic sulfonium salts and iodonium salts with complex anions havethereby proven particularly advantageous. The sulfonium salts canthereby be present alternatively to or together with the iodonium salts.A photo initiator that releases a Lewis acid and is, for examplerealized as a π-donor transition metal complex is also possible.Phenacylsulfonium salts, hydroxyphenylsulfonium salts, and sulfoxoniumsalts are also possibilities. In addition, onium salts that arestimulated to produce acid not directly, but rather by means of asensitizer can also be used. Organic silicon compounds, which release asilanol upon UV radiation in the presence of aluminum organic compounds,can also be used as photo initiators for the cationic hardening process.

The following sulfonium salt is, for example, well suited as a photoinitiator. It is marketed by the Union Carbide Corporation under thetrade name Cyracure UVI 6974. ##STR3##

The thermal initiator component preferably contains a thiolanium saltaccording to the invention, which acts as a thermally activatableinitiator. Benzylthiolanium salts with the following general structureare preferred: ##STR4## whereby R1 is selected from the group consistingof hydrogen, alkyl, aryl, alkoxy, thioether, halogen, CN or NO₂. R2equals hydrogen, alkyl and aryl; R3 is selected from the groupconsisting of hydrogen, alkyl, aryl and an aromatic system condensed onthe thiolane ring; X⁻ is selected from the group consisting of PF₆ ⁻,AsF₆ ⁻ and Sb F₆ ⁻.

Unsubstituted benzylthiolanium salts are thereby preferably used, inparticular benzyithiolanium hexafluoroantimonate.

In addition to the named components, other standard auxiliary materials,such as auxiliary wetting agent, auxiliary flowing agent, bonding agent,thixotropic agent, anti-foaming agent, coloring, etc. may be containedin the inventive casting resin formulation. In the following, theinvention is described in more detail on the basis of a possibleexemplary embodiment, which is not intended to limit the scope of theclaimed subject matter of the invention. All indications are in weight%.

Casting resin formulation, comprising:

29.1 cycloaliphatic diepoxide (CY 179, from the Ciba-Geigy)

3.30 Isocyanurate siloxane (Baysilone SSA 762, Bayer AG)

0.17 Glycidyloxypropylthrimethoxy siloxane (Silan A-187, UCC)

0.03 Acrylic resin (Modaflow, Monsanto)

0.17 Triarylsulfoniumhexafluoroantimonate (Cyracure UVI 6974)

0.23 Benzylthiolaniumhexafluorontimonate (PI 55, Aldrich) and

67.0 Quartz material (FB-3 S(X), Denka Company)

For the manufacture of an inventive casting resin formulation, theorganic formulation components are dissolved at room temperature and arehomogenized with the magnetic stirrer. The filler material is thendispersed in for 15 minutes at 3500 revolutions per minute using alaboratory dissolver (e.g. Pendraulik LM 34). Degassing then takes placefor 15 minutes at 0.2 mbar with stirring using a vane stirrer. Thecomposition produced in this way can be stored at room temperature forat six months, and can be dosed using a pressure/time dispenser with orwithout pre-heating.

The inventive resins display the advantages of storability at roomtemperature, better underfill behavior and UV hardenability.

With the inventive compound, 20 μm gaps in a chip substrate can befilled in a few minutes at a high temperature (e.g., 60 to 90° C.,preferably 75° C.); the hardening can thereby be initiated by UVradiation and/or thermally.

The hardening ensues after the UV initiation (1500 mJ; the sealingcompound is then already hard and the component can be handled withoutdifficulty) by means of thermal after hardening (e.g., 15 minutes at150° C.). However, hardening can also take place exclusively thermally,i.e., without UV-initiated pre-hardening (e.g., 16 hours at 130°, or 6hours at 140°, or 3 hours at 1500).

The inventive molding materials hardened in this way have characteristicvalues [expansion coefficients (25 ppm/K), stress behavior in thebending beam test (122 rel. Units), temperature cycle strength (-40°C./125° C., 100 cycles)] that are comparable to those from the priorart.

With respect to absorption of humidity and E-corrosion behavior, theinventive molding compounds fulfil the requirements for chip coveringcompounds, with 0.3% (3 d 23° C., distilled water) and AN 1.2 (VDE0303).

In the following, the invention is explained in more detail on the basisof a schematic Figure.

In the Figure, the substrate 1 is drawn in at the bottom with hatching,and above this the chip 2 is shown without hatching. The substrate 1 andchip 2 are connected with one another by means of several solderedcontacts 3. The height of these soldered points, e.g., 20 μm, determinesthe height of the gap. When the sealing compound is dosed along the edgeof the chip in front of the gap, the compound underflows the gap due tocapillary action.

With the formulation indicated above in the example, a 20 μm gap can beunderfilled with resin within 10 minutes at a substrate temperature of75° C. For comparison, let it be noted that with the best commerciallyavailable sealing compounds, such as for example Hysol FP 4511, the 20μm gap could be filled only to about 20% with partial filling materialfiltration under the conditions indicated above.

It is thus clear that the inventive casting resin formulation not onlycomprises essentially better storage life, but also shows betterperformance in use, such as e.g. gap filling behavior and processingtemperature. In addition, it can thus be seen that the homogeneity ofthe inventive formulations is superior to that of the sealing compoundsin the prior art.

The inventive resins show the advantages of storability at roomtemperature, better underfill behavior and UV hardenability. Storagestability thereby relates to a time of 6 months.

From the above description, it is apparent that the objects andadvantages of the present invention have been achieved. While onlycertain embodiments have been set forth, alternative embodiments andvarious modifications will be apparent from the above description tothose skilled in the art. These and other alternatives are consideredequivalents and within the spirit and scope of the present invention.

What is claimed:
 1. An electronic component comprising:a substratelayer, a chip disposed on top of a substrate, the chip being connectedto the substrate by a plurality of soldered contacts disposed between anunderside of the chip and a top surface of the substrate, the solderedcontacts defining a gap between an underside of the chip and the topsurface of the substrate, the gap accommodating a casting resinformulation comprisingan epoxy resin; an isocyanurate siloxane a fillermaterial having a grain size of less than 20 μm; a photo initiator; anda thermal initiator.
 2. The electronic component of claim 1 wherein theepoxy resin is present in an amount ranging from about 15% to about 75%by weight,the siloxane is present in an amount ranging from about 0.2%to about 15% by weight, the filler material is present in an amountranging from about 25% to about 85% by weight, the photo initiator ispresent in an amount ranging from about 0.01% to about 5% by weight, andthe thermal initiator is present in an amount ranging from about 0.01%to about 5% by weight.
 3. The electronic component of claim 1 whereinthe epoxy resin is further characterized as being a cycloaliphatic epoxyresin.
 4. The electronic component of claim 1 wherein the photoinitiator is further characterized as being a triarylsulfonium salt. 5.The electronic component of claim 1 wherein the thermal initiator isfurther characterized as being a benzylthiolanium salt.
 6. An electroniccomponent comprising:a substrate layer, a chip disposed on top of asubstrate, the chip being connected to the substrate by a plurality ofsoldered contacts disposed between an underside of the chip and a topsurface of the substrate, the soldered contacts defining a gap betweenan underside of the chip and the top surface of the substrate, the gapaccommodating a casting resin formulation comprising an epoxy resin; asiloxane; a filler material having a grain size of less than 20 μm andwherein the grains of the filler material have a spherical shape; aphoto initiator; and a thermal initiator.
 7. The electronic component ofclaim 6 wherein the siloxane is further characterized as being anisocyanurate siloxane.